This invention relates to glasses of low lead oxide content useful for the production of electrical devices.
In electrical devices such as, for example, light bulbs, the envelope of these lamps (incandescent or fluorescent type) is generally composed of an inexpensive glass of the soda-lime type, whereas that part of the lamp supporting the electrical conductors and permitting the evacuation of air from the envelope and subsequent filling with gas is currently produced from a glass of the R.sub.2 O-PbO-Al.sub.2 O.sub.3 -SiO.sub.2 (R=Na and K) type containing a large percentage of lead oxide (between 20-25% by weight). Other than the fact that this glass can be sealed to the glass of the envelope and to the electrical conductors, it exhibits a high volume electrical resistivity in order to insure a proper insulation between the conductor wires during operation of the lamp. The envelope glass (soda-lime glass) does not satisfy this last property.
From the standpoint of production and use of the bulbs, the present glasses are perfectly satisfactory. However, the presence of a high lead oxide content involves some disadvantages which one would wish to minimize. To illustrate, the preparation and melting of a glass containing more than 20% lead oxide can lead to dusting of the batch and volatilization of toxic lead compounds which problems require precautions and special equipment in production. On the other hand, the price of the starting material, the source of lead oxide, is high which leads to an expensive glass forming batch. For these reasons, research was conducted to replace the glass of high PbO content with a glass containing no PbO or very little PbO, but which exhibits essentially the same properties. The object of the present invention is to provide such a glass.
The principal properties that the glass should possess, and which also define the lead glasses currently used, are the following: a coefficient of thermal expansion between 25.degree.-300.degree. C. less than about 96.5.times.10.sup.-7 /.degree.C. in order to be capable of sealing to the electrical conductors of DUMET alloy; a softening point or Littleton point (T.sub.L) corresponding to a viscosity of 10.sup.7.6 poises less than about 660.degree. C; a volume electrical resistivity at 250.degree. C. greater than 10.sup.8.5 ohm cm; and a viscosity at the highest temperature of devitrification or liquidus greater than about 50,000 poises in order to be able to form the glass without problems of devitrification with current procedures, e.g., drawing tube or cane. For the glasses of the invention, this viscosity corresponds approximately to a temperature of 900.degree. C. Thus, the selected glasses exhibit a liquidus temperature less than 900.degree. C.
Hence, the problem to be resolved consists in finding a region of compositions which, when the PbO content is greatly reduced, permits the formulation of a low cost glass forming batch with the properties mentioned above. Also, research was conducted to obtain a softening point as close as possible to that of the glasses currently used (.about.630.degree. C.) in order not to profoundly modify the sealing conditions.